The specific aims of this study are: (1) to measure baseline bone turnover by serum and urine biochemical markers in men over age 65 years with low femoral neck bone density; (2) to measure biochemical markers of bone turnover after 9 weeks treatment with 17-B estradiol; (3) to examine the relationship between baseline bone turnover and bone mass at the femoral neck, lumbar spine and total body in older men; and, (4) to examine the response of calcium regulating and sex hormones to low dose estrogen replacement therapy in older men. Five healthy men with low bone density at the femoral neck were given 0.5mg micronized 17-B estradiol x 9 weeks, and followed for a total of 18 weeks. The result was that no significant effect or trend was seen on markers of bone turnover. Due to these results, seven men were treated with a full replacement dose of 17-b (2.0mg) for nine weeks. These men were followed for an additional 9 weeks off therapy in 6 of the seven men. The remainder of the data is currently being analyzed.